01 NRDC Dyslexia 1-88 update - Texthelp

20
Research Report
The British Dyslexia Association’s prevalence estimates of either 4 per cent for severe
dyslexia or 10 per cent to include mild dyslexia have been described as ‘both theoretically and
technically contentious’ (Pumfrey, 2001). Lower, but possibly no less contentious, are the
estimate of 2.29 per cent for (what was then called) ‘specific reading retardation’ obtained
from the Child Health and Education Study cohort (Rodgers, 1983) and the figure of 2.08 per
cent given not as a prevalence estimate but as representing ‘the children in the cohort about
whose dyslexia one can have the most confidence’ (Miles et al., 1993). In the original
standardisation sample for the British Ability Scales (BAS), 8.5 per cent were defined as
showing general reading backwardness, while a further 3.2 per cent were defined as showing
‘specific reading retardation’ (Tyler & Elliott, 1988). Because ‘specific reading retardation’ is a
wider concept than developmental dyslexia, the prevalence of dyslexia must have been less
than 3.2 per cent in the BAS standardisation sample.
However, where dyslexics are identified by IQ-discrepancy methods, prevalence estimates
may be particularly sensitive to teaching techniques; reading difficulty rates are higher with
whole-language methods than with systematic and explicit phonics and highest of all among
socially disadvantaged learners (Chapman et al., 2001; Nicholson, 1997). With all such
estimates, unstable diagnoses raise further doubts about their reliability (Badian, 1999;
Haslum, 1989).
It may well be that in asking about prevalence we are posing an inappropriate question. If we
take the perspective that ‘developmental disorders are not diseases that one does or does not
have but are behaviorally defined dimensional traits along a continuum with fuzzy edges and a
wide range of severity’ (Rapin, 2002), then questions about prevalence are misconceived;
‘there is no crisp partition between normalcy and disorder … even when there is no
controversy regarding the identification of prototypic exemplars’ (Rapin, 2002).
None of these difficulties should worry adult literacy tutors. If a reliable estimate of the
prevalence of dyslexia in the general population were ever available, tutors should not expect
to find the same rate among their students, nor is it likely that any student rate would be
stable over place, time and first-language status.
Adult literacy tutors may nevertheless appreciate a word of warning about the high prevalence
sometimes reported for dyslexia among offenders (e.g. Alm & Andersson, 1997; Davies &
Byatt, 1998; Kirk & Reid, 2001; Morgan, 1997; Turner et al., 2000). As the studies in question
do not differentiate dyslexics from ordinary poor readers, their prevalence estimates must be
inflated (Rice, 2000; Samuelsson et al., 2003; Snowling et al., 2000). A history of childhood
disadvantage is more frequent among prisoners than it is in the general population (Dodd &
Hunter, 1992; Social Exclusion Unit, 2002); between childhood disadvantage and low literacy,
a causal association is well-documented (Cox & Jones, 1983; Davie et al., 1972; Kolvin et al.,
1990; MacKay, 1999; Mortimore & Blackstone, 1982; Nicholson, 1997; Richman, Stevenson, &
Graham, 1982; Tough, 1982). Accordingly, it is reasonable to expect that prisoners’ reading
skills will reflect their social circumstances and ability levels (Putnins, 1999), which is what
systematic enquiries have found (Black et al., 1990; Haigler et al., 1994).
In passing, tutors are advised that the frequently cited ALBSU study is an unreliable guide to
prisoner literacy, because of its haphazard sampling and poor data quality — problems that
were not reported on publication (ALBSU, 1994).
After social and educational background variables have been taken into account, there